You can add multiple secondary subnets on the loopback used as unnumbered source. Be sure to add them to the router configuration before adding them to the dhcp server so that the default gw and routing will work for the subscribers in the new subnet...
According to the RSP880 datasheet at https://www.cisco.com/c/en/us/products/collateral/routers/asr-9000-series-aggregation-services-routers/datasheet-c78-733763.html it seems like the only supported 5th gen card is the 8HG, not the 20HG.
To explicitly negate the shutdown node, which I suspect is a leaf of type "empty" - which is why <shutdown>false</shutdown> doesn't work, you'd need to send the node with a delete function, something like:<shutdown xmlns:nc="urn:ietf:params:xml:ns:ne...
NCS540 is built on Jericho Qumran-AX with 3GB of packet buffer, as compared to the ASR920 that has around 10-12MB or something the like, so that should not be a problem. See for example https://www.ciscolive.com/c/dam/r/ciscolive/apjc/docs/2019/pdf/B...
